Hello Livernoiselmhurst,
Based on the information provided at Create and edit events in Calendar on iPhone - Apple Support, these steps should let you add an attachment using Calendar:
Add an attachment
You can add an attachment to a calendar event to share with invitees.
1. Tap the event, then tap Edit near the top right.
2. In the event details, tap Add attachment.
3. Locate the file you want to attach.
To find the file, you can enter its name in the search field, scroll, tap folders to open them, tap Browse to look in other locations (such as iCloud Drive), and so on. See View and modify files and folders in Files on iPhone.
4. Tap Done.
To remove the attachment, tap the event, tap Edit near the top right, swipe left over the attachment, then tap Remove.
If you would like to suggest a feature to Apple developers, since these forums are users helping other users, the best way to do that is to provide feedback to them here: Product Feedback - Apple
Have a good day.